In recent years, in order to increase the capacity of an optical disk, blue shift of laser light to be employed for writing and reading of information is in progress. At present, laser light having a wavelength of 780 nm is used for CDs, and that of 660 nm is used for DVDs, but it has been studied to use laser light having a wavelength of from 300 to 450 nm for optical recording media of the next generation. Accordingly, an optical element such as a diffraction element or a phase plate to modulate laser light having a wavelength of from 300 to 450 nm (hereinafter referred to also as blue laser light), is required, and therefore, an optical anisotropic material useful for laser light in such a wavelength range is needed.
On the other hand, liquid crystal molecules having polymerizable functional groups have both characteristics as a polymerizable monomer and characteristics as a liquid crystal. Accordingly, if the polymerization reaction is carried out after liquid crystal molecules having polymerizable functional groups are aligned, it is possible to obtain an optical anisotropic material having alignment of the liquid crystal molecules fixed. The optical anisotropic material has an optical anisotropy such as a refractive index anisotropy derived from a mesogen skeleton and is applied to e.g. a diffraction element or a phase plate by the use of such a characteristic.
As such an optical anisotropic material, for example, a polymer liquid crystal has been reported which is obtained by polymerizing a liquid crystal composition containing a compound represented by the following formula (2):
(wherein Q is a 1,4-phenylene group or a trans-1,4-cyclohexylene group, and Z is an alkyl group) (see JP-A-10-195138).
Further, the following properties may be mentioned as properties usually required for an optical anisotropic material for a diffraction element or a phase plate.
(1) Absorption of light is low.
(2) The in-plane optical properties (such as the retardation value) are uniform.
(3) The optical properties can easily be adjusted to those of other materials constituting the element.
(4) The wavelength dispersion of the refractive index is small.
(5) The durability is good.